Shelf



FIG. 1 is a perspective view of the shelf showing my new design;

FIG. 2 is an enlarged front view;

FIG. 3 is an enlarged top view;

FIG. 4 is an enlarged side view;

FIG. 5 is an enlarged rear view;

FIG. 6 is an enlarged bottom view. 

The ornamental design for a shelf, as shown. 